The Committee Consultative International Telephone and Telegraphique (CCITT) has drafted Standards for Integrated Services Digital Networks (ISDN); the so-called I-Series Recommendations. These Recommendations include provision of two B-channels and a D-channel transmitted in patterns ("frames") of a specified structure. The structure includes a framing "F" signal bit which precedes each frame.
The signals on the B- and D-channels are received in frames at a four-wire interface, or reference point, from a network by one of several terminals (terminal equipment or "TE") connected along a transmission line. Since the signal strength decays with distance along the line, and because the CCITT standards specify fixed threshold signal strengths to represents an active line, the TE's require "adaptation" to compensate for the decaying signal. Besides signal decay on the line, the presence of multiple transmitting terminals on the line results in voltage addition which can effect the amplitude of the signal received by the TE.
Various "adaptive threshold" schemes are known in the prior art to allow a wide dynamic range in the signals present on the transmission line. For instance, sampling of the F-bit signal strength is commonly used. However, because of relaxed noise margin, the signal strength of the F-bit can be as much as 160% of a nominal value and thus the F-bit is not the best indicator of signal strength on which to base adaptation. Other schemes use the D-channel signal strength, but because a number of terminals can simultaneously transmit D-channel data, until a "collision" occurs, D-channel signal strength similarly is not the best basis for adaptation.